The Sea Surface Microlayer (SSM) or Sea Surface Layer (SSL) is defined as the top 1 micron > 1mm 'layer' of the Earth's oceans. It's thought that the layer behaves differently to the main body of the ocean, acting as the 'interface' between the atmosphere and the seawater beneath.

The SSM provides "an aggregate-enriched biofilm environment with distinct microbial communities.". It also has different salinity levels - the fresher water, topped up by rainfall, tends to 'float' on the more dense, saltier water below it. The lower salinity allows micro-organisms to create a different 'organic matrix'.

The properties of the layer, and its implications for gas-exchanges and possible climate-changing effects, are 'poorly researched'.

As the ocean covers 362 million km² (~71%) of the Earth's surface, the ocean-atmosphere interface is arguably one of the largest and most important interfaces on the planet.
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Like a skin, the SML is expected to control the rates of exchange of energy and matter between air and sea, thereby potentially exerting both short-term and long-term impacts on various Earth system processes, including biogeochemical cycling and climate regulation (Cunliffe et al., 2013). Yet, processes occurring within the SML, as well as the associated rates of material exchange through the SML, are still poorly understood.

Source : Frontiers in Marine Science. 4